Genotype vs phenotype for GCSE Biology
Your genotype is the combination of alleles you carry for a particular gene. Your phenotype is the observable characteristic that results from that genotype, often combined with environmental factors. In short: Genotype is the genetic code; phenotype is the trait you can see.
This guide covers the precise definitions AQA expects, dominant and recessive alleles, homozygous and heterozygous combinations, how to read a Punnett square, and the wording the mark scheme rewards on the higher-tier GCSE Biology inheritance questions.
Genotype: The alleles you have
Written as letters such as BB, Bb or bb. Capital letter for dominant allele, lowercase for recessive. Two alleles per gene, one from each parent.
Phenotype: The trait you show
The physical or biochemical characteristic produced by the genotype, often shaped by the environment. Brown eyes, blood group A, attached earlobes.
Dominant beats recessive
A dominant allele is expressed even when only one copy is present. A recessive allele only shows when both copies are present (homozygous recessive).
Defining genotype properly
The full mark-scheme definition of genotype is: The combination of alleles an organism has for a particular gene (or genes). Genotype is written with two letters, one for each allele inherited from each parent.
For a single gene with a dominant and recessive allele, there are three possible genotypes. BB is homozygous dominant (two dominant alleles). Bb is heterozygous (one of each). bb is homozygous recessive (two recessive alleles).
Defining phenotype properly
The full mark-scheme definition of phenotype is: The observable characteristics of an organism, produced by the interaction of its genotype with the environment. Phenotype is what you can see, measure or detect, such as eye colour, height, blood group or whether a plant has purple or white flowers.
Phenotype is not always purely genetic. Height, for example, depends on both your genotype and environmental factors such as nutrition during childhood. Eye colour, by contrast, is almost entirely determined by genotype (it is actually controlled by several genes working together, but at GCSE it is usually treated as a single-gene example for simplicity).
A common mark-scheme phrase Use 'the observable characteristics produced by the genotype' rather than 'what something looks like'. The first phrasing is precise and earns the mark. The second is too casual and risks losing it.
Dominant, recessive, homozygous, heterozygous
Four key vocabulary terms appear on almost every inheritance question. Learn the precise definitions, because AQA mark schemes are strict about wording.
| Term | Definition | Example |
|---|---|---|
| Dominant | An allele expressed in the phenotype even when only one copy is present | B (brown eyes) |
| Recessive | An allele only expressed in the phenotype when two copies are present | b (blue eyes) |
| Homozygous | An organism with two identical alleles for a gene (BB or bb) | BB or bb |
| Heterozygous | An organism with two different alleles for a gene (Bb) | Bb |
Worked example: A monohybrid cross
In pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p). A heterozygous purple-flowered plant (Pp) is crossed with a homozygous white-flowered plant (pp). What are the expected genotypes and phenotypes of the offspring?
Step 1: Draw a Punnett square with parent gametes on the outside. Parent 1 (Pp) produces P and p gametes. Parent 2 (pp) produces only p gametes.
Step 2: Fill in the squares: Pp, Pp, pp, pp.
Step 3: Read off the ratios. Genotype ratio is 2 Pp : 2 pp, or 1:1. Phenotype ratio is 2 purple : 2 white, or 1:1.
Full-mark answer: 50% of offspring are heterozygous (Pp) with purple flowers, and 50% are homozygous recessive (pp) with white flowers. The expected ratio is 1 purple : 1 white.
Probabilities are predictions, not guarantees A Punnett square gives the expected ratio over a large number of offspring. With only four offspring, you could easily see three purple and one white, or even four of one colour. Always write 'expected' ratio in your answer.
How environment affects phenotype
Many phenotypes are shaped by both genotype and environment. Height is a classic example: Two identical twins with the same genotype can grow to different heights if one has better nutrition during childhood.
Other examples include skin colour (genotype plus sun exposure), body mass (genotype plus diet and exercise), and even some flower colours that change with soil pH. AQA expects you to give one named example of how the environment can affect phenotype.
Genotype, phenotype and inheritance diagrams
Inheritance questions often ask you to label both genotype and phenotype on a family tree or Punnett square. The convention is to write the genotype in letters (Bb) and the phenotype in words below or next to it (brown eyes).
A pattern AQA likes to test: A condition that skips a generation in a family tree is usually caused by a recessive allele. Both grandparents must be carriers (heterozygous, Bb), and the affected grandchild must be homozygous recessive (bb).
Where students lose marks
AQA examiner reports flag the same handful of vocabulary slips every year on genotype and phenotype questions. Most are about wording, not the underlying biology.
Common mistakes that cost easy marks Mixing up genotype and phenotype in the answer. Writing the dominant allele in lowercase or the recessive in capitals. Saying a heterozygous organism 'has the recessive phenotype' (it has the dominant phenotype because the dominant allele is expressed). Forgetting that ratios from Punnett squares are expected ratios, not guaranteed outcomes. Saying 'a gene for blue eyes' when you mean 'an allele for blue eyes'.
Key facts to memorise
- Genotype: The combination of alleles an organism has for a gene
- Phenotype: The observable characteristics produced by the genotype
- Allele: A different version of a gene (e.g. B for brown, b for blue)
- Homozygous: Two identical alleles for a gene (BB or bb)
- Heterozygous: Two different alleles for a gene (Bb)
- Dominant: Expressed with one or two copies; written in capital letters
- Recessive: Only expressed with two copies; written in lowercase
- Phenotype can be shaped by both genotype and environment
